1. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Simultaneous Measurements of NO 2 and its Dimer N 2 O 4 at Room Temperature with a Multiplexed Intra-pulse Quantum Cascade Laser Spectrometer Geoffrey Duxbury, David Wilson and Nigel Langford Department of Physics (SUPA), University of Strathclyde, John Anderson Building, 107 Rottenrow, Glasgow, G4 0NG, UK g.duxbury@strath.ac.uk

2. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Outline of Talk Multiplexed Intra-pulse Quantum Cascade laser spectrometer Dimerisation of NO 2 to N 2 O 4 FT infrared spectrum of N 2 O 4 Simultaneous measurements with and without N 2 O 4 Expanded view of NO 2 -N 2 O 4 QC laser spectra Concentrations of NO 2 -N 2 O 4 at room temperature Summary

3. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Spectrometer using pulsed quantum cascade lasers: Multiplex Method Apply a 1500 ns top hat current pulse to each DFB QC laser, with a time delay of the trigger pulses. Obtain a light pulse in time domain with two separated frequency down chirps. Pass pulse through absorbing species and monitor pulse absorption in time domain. Laser and Vigo MCT detector both Peltier cooled, no liquid nitrogen needed Layout of QC lasers housed in Cascade Technologies “Developer Heads”. Each with a ZnSe lens to collimate the output beam.

4. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Multiplexed spectra, preset time delay between the two pulse generators for QC laser1 and QC laser 2. Averaged output from Acqiris digitiser LHS digitised output from Vigo detector, RHS Converted to transmission

5. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Temperature dependence of the N 2 O 4 - NO 2 system Calculated temperature dependence of the N 2 O 4 - NO 2 equilibrium pressures for an overall pressure of 1 Torr. After I.A. Leenson, J. Chemical Education 77, 1652- 1655 (2000 )

6. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m FTIR spectra of N 2 O 4 the region of 11 Recorded at ULB and ETH

7. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m N 2 O 4 11 band at 293.15 K (from Jean Vander Auwera, ULB) dh11, S band = 5.93(64) (in 10 -17 cm/molecule) J. Quant. Spectrosc. Radiat Transfer 50, 595 (1993)

8. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Absorbance spectra of NO 2 recorded in the 1276 cm -1 region (a) and the 1343 cm -1 region (b) Region (a) lies close to the 11 band of N 2 O 4. The displacement of the baseline with pressure is evidence of overlap with the wing of the R branch of the 11 band.

9. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Absorbance spectra of NO 2 recorded higher gas pressures (a) 1276 cm -1 region and (b) the 1343 cm -1 region Note the increasing baseline shift with pressure in (a), and also the increasing number of sharp absorption features of N 2 O 4. No baseline shifts are observed in (b)

10. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Increased absorbance of the quasi-continuum associated with the  11 band of N 2 O 4, as the scan is shifted towards the N 2 O 4 band centre.

11. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Lowest wavenumber spectrum of NO 2 using an operating temperature of 43 C and a pulse length of 2  s. The lowest wavenumber achieved, 1270.1 cm -1, is approximately 9 cm -1 above the centre of the 11 band of N 2 O 4,1261.1 cm -1.

12. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Comparison of FTS and QCL 1 spectra at the high and low wavenumber regions of QCL 1 High wavenumber Low wavenumber

13. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Summary In this paper we have shown that long duration pulse operation of the recent generation of QC lasers provides a powerful tool for measuring complex spectra, such as those provided by the NO 2 - N 2 O 4 equilibrium. The resolution is similar to a high resolution Fourier transform spectrometer, see [1], and allows both sharp and broad absorption features to be measured simultaneously. Comparison of two regions, one with overlapping NO 2 and N 2 O 4, the other with NO 2 only, allows the equilibrium to be determined at a particular temperature. In a previous optical measurement of this equilibrium only the electronic spectrum of NO 2 was used. (L. Harris and K.L. Churney, J. Chem. Phys. 47, 1703-1709 (1967)

14. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m [1] G. Duxbury N. Langford. K G. Hay and N. Tasinato, ” Quantum cascade laser spectroscopy: diagnostics to non-linear optics ”, J. Mod. Opt, 56, 2034-2048 (2009) [2] D. Hurtmans, M. Herman and J. Vander Auwera, “ Integrated band intensities in N 2 O 4 in the infrared range ”, J. Quant. Spectrosc. Radiat. Transfer 50, 595-602 (1993) [3] D. Luckhaus and M. Quack, “ High-resolution FTIR spectra of NO 2 and N 2 O 4 in supersonic jet expansions and their rovibrational analysis ”, Chem. Phys. Lett. 199, 293-301 (1992) [4] M. Hepp, R. George, M. Herman, J.-M. Flaud and W.J. Lafferty, “ Striking anharmonic resonances in N 2 O 4 : supersonic jet Fourier transform spectra at 13.3, 7.9, 5.7 and 3.2  m ”, J. Mol. Struct., 517-518, 171-180, (2000) [5] F. Melen, F. Pokorni and M. Herman, ” Vibrational band analysis of N 2 O 4 ” Chem. Phys. Lett., 194 181-186 (1992) [6] Y. Elyoussoufi, M. Herman, J. Lievin anf I. Kleiner, “ Ab initio and experimental investigation of the vibrational energy pattern in N 2 O 4 : the mid and near infrared ranges ”, Spectrochim. Acta, A53, 881-894 (1997) [7] A. Perrin, J.-Y. Mandin, C. Camy-Peyret, J.-M. Flaud, J.-P. Chevillard and G. Guelachvili, “ The 1 band of 14 N 16 O 2 : Line positions and intensities ”, J. Mol. Spectrosc. 103, 417-435 (1984) [8] A. Perrin, J.-M. Flaud, C. Camy-Peyret, A.-M. Vasserot, G. Guelachvili, A. Goldman, F.J. Murcray and R.D. Blatherwick, “ The 1, 2 2 and 3 interacting bands of 14 N 16 O 2 : Line positions and intensities. ”, J. Mol. Spectrosc., 154, 391-406 (1992) [9] L.S. Rothman et. al. “ The HITRAN 2008 molecular spectroscopic database ”, J. Quant. Spectrosc. Radiat. Transfer. 110, 533-572 (2009) References

15. June 19th 2012 160 MATH. ANNEX Atmos. Spec. TA3 9:04 a.m Acknowledgements We are indebted to the EPSRC for for the award to David Wilson of a studentship through the Doctoral Training Fund GD is grateful to the Leverhulme Trust for the award of an Emeritus Followship We would also like to thank the Jean Vander Auwera, and his colleagues at the ULB, Bruxelles, for providing the Fourier transform spectra of N 2 O 4 used in this presentation